Low loss synchronous rectifier for application to clamped-mode power converters

Abstract

A synchronous rectifier for use with a clamped-mode power converter uses in one embodiment a hybrid rectifier with a MOSFET rectifying device active in one first cyclic interval of the conduction/nonconduction sequence of the power switch and a second rectifying device embodied in one illustrative embodiment as a low voltage bipolar diode rectifying device active during an alternative interval to the first conduction/nonconduction interval. The gate drive to the MOSFET device is continuous at a constant level for substantially all of the second interval which enhances efficiency of the rectifier. The bipolar rectifier device may also be embodied as a MOSFET device. The subject rectifier may be used in both forward- and flyback-type power converters.

What is claimed is: 
     
       1. A power converter, comprising: an input for accepting a DC voltage;   an integrated magnetic device having a primary winding and a secondary winding;   a power switch for periodically connecting the input to the primary winding;   an output for accepting a load to be energized;   a clamping circuit for limiting a voltage across the secondary winding during a first interval of a cyclic period of the power converter; and   a rectifier circuit connecting the secondary winding to the output, the rectifier circuit, comprising: a synchronous rectification device with a control terminal connected to be responsive to a signal across the secondary winding such that the synchronous rectification device conducts a load current during the first interval; and   a diode connected for enabling conduction of the load current during a second interval other than the first interval.     
     
     
       2. The power converter as recited in claim 1 wherein the integrated magnetic device has a transformer portion and an inductor portion. 
     
     
       3. The power converter as recited in claim 1 wherein the rectifier circuit further comprises a first gate voltage limiting device coupled to the synchronous rectification device. 
     
     
       4. The power converter as recited in claim 1 wherein the power converter is constructed to operate as a forward-type converter. 
     
     
       5. The power converter as recited in claim 1 wherein the power converter is constructed to operate as a flyback-type converter. 
     
     
       6. A power converter, comprising: an input for accepting a DC voltage;   an integrated magnetic device having a primary winding and a secondary winding;   a power switch for periodically connecting the input to the primary winding;   an output for accepting a load to be energized;   a clamping circuit for limiting a voltage across the secondary winding during a first interval of a cyclic period of the power converter; and   a rectifier circuit connecting the secondary winding to the output, the rectifier circuit, comprising: a first synchronous rectification device with a control terminal connected to be responsive to a signal across the secondary winding such that the synchronous rectification device conducts a load current during the first interval; and   a second synchronous rectification device with a control terminal connected to be responsive to a signal across the secondary winding such that the second synchronous rectification device conducts the load current during a second interval other than the first interval.     
     
     
       7. The power converter as recited in claim 6 wherein the integrated magnetic device has a transformer portion and an inductor portion. 
     
     
       8. The power converter as recited in claim 6 wherein the rectifier circuit, further comprises: a first gate voltage limiting device coupled to the first synchronous rectification device; and   a second gate voltage limiting device coupled to the second synchronous rectification device.   
     
     
       9. The power converter as recited in claim 6 wherein the power converter is constructed to operate as a forward-type converter. 
     
     
       10. The power converter as recited in claim 6 wherein the power converter is constructed to operate as a flyback-type converter. 
     
     
       11. A switching mode power converter, comprising: an integrated magnetic device having a primary winding and a secondary winding, the integrated magnetic device further having a magnetizing inductance therein requiring periodic recycling;   a first power stage for converting a DC input into a periodic pulsed voltage applied to the primary winding of the integrated magnetic device, the first power stage having a clamping circuit for limiting a voltage of the integrated magnetic device during the periodic recycling; and   a second power stage for rectifying an output of the secondary winding of the integrated magnetic device and applying it to a load to be energized, the second power stage having a synchronous rectifier having a rectifying device with a control gate connected to be responsive to a signal across the secondary winding such that the rectifying device conducts a load current during the periodic recycling when the clamping circuit is active, the second power stage further having a diode connected for enabling conduction of the load current when the rectifying device is nonconducting.   
     
     
       12. The switching mode power converter as recited in claim 11 wherein the integrated magnetic device has a transformer portion and an inductor portion. 
     
     
       13. The switching mode power converter as recited in claim 11 wherein the synchronous rectifier further comprises a gate voltage limiting device coupled to the rectifying device. 
     
     
       14. The switching mode power converter as recited in claim 11 wherein the secondary winding is tapped and separated into a first and a second winding segment, the first winding segment thereby connected to the rectifying device and the second winding segment thereby connected to the diode. 
     
     
       15. The switching mode power converter as recited in claim 11 wherein the switching mode power converter is constructed to operate as a forward-type converter. 
     
     
       16. The switching mode power converter as recited in claim 11 wherein the switching mode power converter is constructed to operate as a flyback-type converter. 
     
     
       17. A switching mode power converter, comprising: an integrated magnetic device having a primary winding and a secondary winding, the integrated magnetic device further having a magnetizing inductance therein requiring periodic recycling;   a first power stage for converting a DC input into a periodic pulsed voltage applied to the primary winding of the integrated magnetic device, the first power stage having a clamping circuit for limiting a voltage of the integrated magnetic device during the periodic recycling; and   a second power stage for rectifying an output of the secondary winding of the integrated magnetic device and applying it to a load to be energized, the second power stage having a synchronous rectifier having a first rectifying device with a control gate connected to be responsive to a signal across the secondary winding such that the first rectifying device conducts a load current during the periodic recycling when the clamping circuit is active, the synchronous rectifier further having a second rectifying device with a control gate connected to be responsive to a signal across the secondary winding such that the second rectifying device conducts load current when the first rectifying device is nonconducting.   
     
     
       18. The switching mode power converter as recited in claim 17 wherein the integrated magnetic device has a transformer portion and an inductor portion. 
     
     
       19. The switching mode power converter as recited in claim 17 wherein the synchronous rectifier, further comprises: a first gate voltage limiting device coupled to the first rectifying device; and   a second gate voltage limiting device coupled to the second rectifying device.   
     
     
       20. The switching mode power converter as recited in claim 17 wherein the secondary winding is tapped and separated into a first and a second winding segment, the first winding segment thereby connected to the first rectifying device and the second winding segment thereby connected to the second rectifying device. 
     
     
       21. The switching mode power converter as recited in claim 17 wherein the switching mode power converter is constructed to operate as a forward-type converter. 
     
     
       22. The switching mode power converter as recited in claim 17 wherein the switching mode power converter is constructed to operate as a flyback-type converter.